The present invention relates to trellis-coded modulation techniques and, more particularly, the use of these techniques in fading channel applications, e.g., in cellular mobile radio.
For communications systems that contain AWGN channels, i.e., channels that are characterized by additive white Gaussian noise (AWGN), trellis-coded modulation methods have been found to provide a "coding gain" in signal power (compared to so-called "uncoded" modulation methods) with the result of improving the error performance of the system without requiring additional bandwidth. For example, trellis-coded modulation has proved to be a practical power-efficient and bandwidth-efficient modulation method for use in commercial telephone-line modems, i.e., data communications, and has resulted in an increase of line rates of those modems to as much as 19.2 Kbits/sec.
Trellis-coded modulation can be classified into two categories: two-dimensional (2 D) trellis-coded modulation and multidimensional trellis-coded modulation. Those working in the prior art have shown that when applied to an AWGN channel multidimensional trellis-coded modulation has many advantages over 2 D trellis-coded modulation. These advantages are higher coding gains, lower decoder complexities, and smaller constituent 2 D constellations.
Besides the use of trellis-coded modulation in AWGN channels, those in the art have been investigating the applicability of trellis-coded modulation to communications systems that contain "fading" channels, i.e., channels in which the received signal could be too weak to carry any useful information (a phenomenon called "deep fade"). An important example of fading channels is that of cellular mobile radio. In cellular mobile radio, the received signal can change from being very weak to very strong, and vice versa, within only a foot of driving distance (or a few tens of milliseconds at a vehicle speed of 20 miles/hour).
To apply trellis-coded modulation to fading channels, the code should exhibit a so-called "time diversity" property so that the transmitted information may still be recovered in the receiver even in the presence of deep fade. A code is said to have "X-fold" (X.ltoreq.2) time diversity if any pair of valid sequences of signal points of the code differ in at least X signal point positions in the sequence. Conceptually, this time diversity is manifested by an interdependence between the signal points that are produced by the coded modulation scheme. For example, consider the case where two 2 D signal points are produced over a time interval and there is a time-diverse interdependence between these points. As a result of this interdependence, the input data represented by the two 2 D signal points may be able to be accurately recovered even if one of the 2 D signal points is lost. To effectively use the time-diversity property of a code to cope with deep fade, which tends to be bursty, an interleaver is often used to further separate the time intervals in which the signal points that contribute to the diversity of the code are transmitted. The farther these time intervals are spaced, the better the error-rate performance of the coded system is. Generally speaking, for a given interleaver length, which is limited by the total amount of transmission delay that can be tolerated, the shorter the decoding depth (to be described below) of a code is, the farther the signal points that contribute to the X-fold time diversity of the code can be separated and the better the error-rate performance of the code is. Unfortunately, a multidimensional trellis code typically has a longer decoding depth than a 2 D trellis code and, hence, a poorer error-rate performance than a 2 D trellis code when applied to a fading channel.
In consideration of the above, it would be advantageous to minimize the decoding depth of a multidimensional trellis code with X-fold time diversity for application to a fading channel.